Adercotryma kuhnti sp. nov., a Maastrichtian deep-water species from the Pacific Ocean, and evolution of the genus Adercotryma

Adercotryma glomeratum is one of the most geographically and bathymetrically widespread species of Recent deep-sea benthic foraminifera, but the evolutionary history of this distinct agglutinated species is poorly known. In addition to one or two Recent species probably closely related to A. glomeratum, the Eocene-Oligocene species Adercotryma agterbergi has been described, but we suggest that this species be assigned to the genus Ammogloborotalia. A very rare, unnamed species has been described from the Campanian in the western Pacific Ocean, and it has been speculated that the genus originated at about that time in the western Pacific, possibly from the ancestor Ammogloborotalia gyroidinaeformis. We describe the Maastrichtian occurrence of a previously unknown species, which we named Adercotryma kuhnti. This species occurs in the Maastrichtian Abathomphalus mayaroensis planktonic foraminiferal Zone, Micula prinsii nannofossil zone CC26, in the lower bathyal to abyssal ODP Site 1210 (ODP Leg 198, Shatsky Rise, northwest Pacific Ocean), where it constitutes up to 14.7% of the benthic foraminiferal assemblages. We looked for the species in coeval material from many locations in and around the Atlantic Ocean, and DSDP Site 465 in the Pacific (Hess Rise), but did not find it. The occurrence as a geographically restricted species agrees with speculation of a Cretaceous origin of the genus in the western Pacific, and evolution as a deep-sea species rather than migration from shallow waters.